The production of photopolymer reliefs for flexographic printing typically involves imagewise exposing the photosensitive layer of an element with ultraviolet radiation. Unexposed areas are washed off using a suitable solvent. Solvent is removed by evaporation and, if necessary, the surface is treated to remove tack. Examples of this are found in the following Patents and Preliminary Published Applications: U.S. Pat. Nos. 2,948,611, 3,024,180, 3,798,035, 3,951,657, DE 2,138,582, DE 2,215,090, DE 2,223,808 and DE 2,364,873.
Photosensitive compositions used to make these reliefs usually comprise 1) an addition polymerizable, nongaseous ethylenically unsaturated monomer, 2) a photoinitiator or photoinitiating system activated by actinic radiation and 3) a thermoplastic, elastomeric polymeric binder. Generally the binder is a triblock copolymer having an elastomeric block sandwiched between two thermoplastic blocks, e.g., polystyrene-polybutadiene-polystyrene (S-B-S) or polystyrene-polyisoprene-polystyrene (S-I-S). Such block copolymers are preferred over other polymer binders. They are commercially available, cost-effective, are extrudable, show good solvent resistance and are resistant to cold flow. Examples of flexographic printing reliefs containing these block copolymer binders are described in German Patent 2,215,090, German Preliminary Published Application 2,223,808, German Patent 2,815,678 and German Patent 2,610,206.
However, printing reliefs containing block copolymer binders are not always suitable for use in flexographic printing. Such reliefs have decreased flexibility and elasticity which impairs their ability to contact substrates having irregular substrates. Consequently, inferior printed materials are produced. In addition, such printing reliefs become detached from the cylinder on which they are mounted. This is undesirable because it involves a shutdown of machines and production. The inability of the printing relief to remain affixed to the printing cylinder is caused by the insufficient flexibility or rigidity of the printing relief.
The problem arises most frequently with long printing runs and the problem becomes especially acute with printing reliefs affixed to cylinders having small radii. It has been observed that detachment of printing reliefs is promoted by penetration of printing ink solvents into the gap created between the edges of the printing reliefs wound around the printing cylinder.
Use of elastomeric block copolymers as binders in photopolymerizable printing reliefs has been suggested as a way to improve flexibility and elasticity of printing reliefs. This is described in U.S. Pat. No. 4,320,188 and U.S. Pat. No. 4,430,417. Use of polybutadiene as the sole binder is described in European Patent 76,588 and U.S. Pat. No. 4,517,278.
Unfortunately, these alternatives create a new set of problems. Elastomeric binders alone are not suitable to formulate a photosensitive composition to make flexographic printing reliefs because such materials are extremely difficult to extrude due to the high melt viscosity of the elastomer block copolymer. Printing reliefs having polybutadiene as the sole binder exhibit poor wash-off quality due to the poor solubility of polybutadiene in the customary development solutions. Furthermore, these printing elements have cold flow problems which leads to poor storage stability. The surface of the resulting printing relief becomes so weak that it is not suitable for flexographic printing.
Multilayer elements have been tried to improve flexibility and elasticity by using elastic interlayers or underlayers. Such plates are described in U.S. Pat. Nos. 4,162,919, 4,264,705, 4,248,960, 4,266,007.
The disadvantage in using multilayer elements is that a more elaborate manufacturing process is required. This increases the production cost as well as the capacity for error In many instances adhesives or special adhesive layers are required between different layers. Detachment of individual layers from one another can occur and there is no guarantee that the planarity of the printing surface will be maintained.